Last week, I was sifting through some old email, looking for any interesting references to
and I almost overlooked this short note I had written 30+ years ago:
From jeffpar Tue Dec 1 10:26:36 1987 To: scottsc Cc: jeffpar Subject: Re: A20.COM Date: Tue Dec 01 10:32:43 1987 When Nancy and company were doing automated (ie, sgrab) testing of the OS/2 3xbox, keystrokes were getting dropped and duplicated. The problem stems from OS/2's constant mode-switching (from real-mode to protected-mode and back), and the constant disabling and re-enabling of the A20 line. Since OS/2 has to talk to the keyboard controller to do that, it was inter- fering with data coming into the keyboard controller occasionally. (I still don't know why this never happens with IBM keyboards; non-IBM keyboards and sgrab are the only culprits). A20.COM just patches the OS/2 kernel so that it stops fiddling with the keyboard controller to change A20 state, and then enables A20. Since the currently loaded image of OS/2 won't be changing A20 anymore, A20 should remain permanently enabled. This will cause some compatibility problems for apps that rely on the 1meg wrap, which includes apps that use the old "call 5" interface to make DOS calls, but the only one I've run across so far is Word's SPELL utility. | >From scottsc Tue Dec 1 10:09:03 1987 | To: jeffpar | Subject: A20.com | Date: Tue Dec 1 10:07:07 1987 | | | Thanks. Just for curiousity's sake, what is it that A20.com is supposed to | do? Nancys made it sound like it activated some interrupt or another, but she | didn't really know. Other people have told me it has to do with timing. | What's the real reason for running this program?
I barely noticed the last sentence:
This will cause some compatibility problems for apps that rely on the 1meg wrap, which includes apps that use the old "call 5" interface to make DOS calls, but the only one I've run across so far is Word's SPELL utility.
Recently, a handful of people, like Michal over at the OS/2 Museum, and myself, had been looking for examples of old software that used the CP/M-style “CALL 5” interface to DOS, instead of (or in addition to) the “INT 21h” interface.
Here was a prime example, SPELL.COM, that I had found 30 years ago and completely forgotten about.
And what about that
A20 program, you ask? I found a copy of that, too. The
PRO386 was probably the cheap
80386-based PC clone I had bought in 1987, which had a few issues running OS/2. Hence this utility, which apparently
I later adapted to help resolve an issue with OS/2 compatibility testing.
title a20.asm ; A20.ASM ; By Jeff Parsons 8/2/87 ; ; Purpose is to patch standard ROM (actually the copy ; in the PRO386's shadow RAM) to NOT issue commands to the ; keyboard controller that change the A20 address-line state. ; ; Will also check for OS/2 running (real-mode version >= 10) ; and search for, and disable through additional patching, the ; operating system routines that change the A20 state. cseg segment public byte 'CODE' org 100h assume cs:cseg,ds:cseg,es:nothing,ss:cseg main proc far mov ah,30h ;get-version DOS function int 21h sub di,di ;zero if no OS/2 patch to do cmp al,10 ;OS/2 running? jb m1 ;no mov si,offset pattern1 ;yes mov ax,90h mov es,ax ;good segment to start looking in mov cx,offset size1 ;length of pattern1 cld call search ;look for it and leave address in (DI) m1: CLI push es mov ax,0F000h mov es,ax ;check for CLI (FAh) at location 4392h mov dx,offset alreadymsg cmp byte ptr es:[4392h],0C3h je m6 ;ROM already patched call empty8042 ;otherwise, enable A20 now mov al,0D1h out 64h,al ;issue "write output port" command call empty8042 mov al,0DFh ;output port value that enables A20 out 60h,al call empty8042 sub bl,bl ;(BL) = 0 if extended patch required mov dx,offset badrommsg cmp byte ptr es:[00A1h],0Ah je m4 ;likely 8Mhz AT ROM cmp byte ptr es:[00F4h],073h je m4 ;specially modified 6Mhz AT ROM cmp word ptr es:[0FFEAh],4F43h je m4 ;a Compaq ROM cmp word ptr es:[01A6h],0E9FAh jne m6 ;not mine, unknown ROM inc bl ;(BL) = 1 since nominal patch required mov al,3Fh ;fetch reserved byte from CMOS out 70h,al jmp $+2 in al,71h ;got it and al,not 1 ;clear ROM/RAM bit mov dx,270h out dx,al ;make ROM visible jmp $+2 ;then write 1st byte of routine with RET mov byte ptr es:[4392h],0C3h or al,1 ;set ROM/RAM bit out dx,al ;and make RAM visible again m4: mov dx,offset finishedmsg or di,di ;any OS/2 patch to make? jnz m7 ;yes m6: jmp exit m7: pop es cmp byte ptr es:[di-50h],0B0h jne m7a ;resort to grosser patch mov byte ptr es:[di-4Fh],1 jmp short m7b ;more elegant patch, to DisableA20 m7a: mov si,offset patch1 mov cx,offset sizepatch1 rep movsb ;apply the A20COMMON patch (easy part!) m7b: or bl,bl ;full-blown patch required? jne toexit ;no mov si,offset pattern2 ;now look for code in REALMODE mov cx,offset size2 mov dx,offset badpatchmsg call search ;look for it and leave address in (DI) jc toexit ;wasn't found sub cx,cx m8: cmp byte ptr es:[di+13h],0BBh je m9 sub di,3 add cl,3 cmp cl,3 jbe m8 toexit: jmp short exit ;*** SANITY CHECKS *** m9: cmp byte ptr es:[di+48h],0CCh jne exit cmp byte ptr es:[di+61h],00Fh jne exit add di,cx mov ax,es:[di-5] ;yank DOSCODE value out mov bx,es:[di-2] ;yank CSOFFSET RESET value out mov dx,es:[di+4] ;yank DOSGROUP value out mov si,offset patch2 sub di,7 push cx mov cx,offset sizepatch2 rep movsb ;patch #2 complete mov es:[di-4],ax ;plug DOSCODE value into patch add di,13 ;skip to next section pop cx sub di,cx mov ax,es:[di+23] ;yank SAVEAREAPTR value out mov si,offset patch3 mov cx,offset sizepatch3 rep movsb mov byte ptr es:[di+7],0Ah mov es:[di-22],ax ;plug SAVEAREAPTR value into patch mov es:[di-27],bx ;plug CSOFFSET RESET value into patch add di,25 ;skip to final section mov ax,es:[di+1] ;yank SAVEAREA value out mov bx,es:[di+13] ;yank INT_MASK value out mov si,offset patch4 mov cx,offset sizepatch4 rep movsb mov es:[di-16],ax mov es:[di-8],bx mov es:[di-21],dx mov dx,offset completemsg exit: STI mov ah,9 ;print-string DOS function int 21h int 20h ;terminate program main endp search proc near s1: mov ax,di ;see if we've exhausted the segment add ax,cx jc s8 ;this compare will wrap, so exhausted push cx push si push di repe cmpsb pop di pop si pop cx je s9 ;exit if match found inc di ;else try from next byte in segment jmp s1 s8: mov di,0 ;return zero if not found (preserve carry) s9: ret search endp empty8042 proc near sub cx,cx e1: in al,64h ;wait for 8042 input buffer to empty test al,02h ;empty yet? (bit 1 must be zero) loopnz e1 jnz e9 ;if still not empty, drop out of program ret e9: mov dx,offset bad8042msg jmp exit empty8042 endp pattern1 db 0B4h,0DDh,00Ah,0C0h,074h,002h,0B4h,0DFh size1 equ $-pattern1 patch1 db 028h,0C0h,0C3h sizepatch1 equ $-patch1 pattern2 db 060h,06Ah,000h,068h size2 equ $-pattern2 patch2 db 0B8h,040h,000h,08Eh,0D8h,0C7h,006h,069h,000h db 0FFh,0FFh,090h,090h sizepatch2 equ $-patch2 patch3 db 0C7h,006h,067h,000h,0FFh,0FFh,02Eh,0C4h,03Eh,0FFh,0FFh db 026h,089h,025h,026h,089h,045h,002h,090h,090h,090h,090h db 090h,090h,090h,090h,090h,090h,090h,090h,090h sizepatch3 equ $-patch3 patch4 db 0B8h,0FFh,0FFh,08Eh,0D8h,0BEh,0FFh,0FFh,08Bh,024h,08Eh db 054h,002h,0A0h,0FFh,0FFh,0E6h,021h,090h,090h,090h,090h sizepatch4 equ $-patch4 alreadymsg db 'Patch already in place',13,10,'$' badrommsg db 'ROM cannot support patch',13,10,'$' badpatchmsg db 'WARNING: OS/2 patch failed',13,10,'$' bad8042msg db '8042 time-out error',13,10,'$' completemsg db 'OS/2 Patched',13,10 finishedmsg db 'Patching complete',13,10,'$' cseg ends end main
Break The SPELL
So, for your debugging pleasure, I’ve set up an IBM PC (Model 5160)
to demonstrate Microsoft’s
SPELL.COM use of “CALL 5”. It’s configured as follows:
id: ibm5160 type: pcx86 debugger: true uncompiled: true commands: bp &060E:0005 "dh;h" config: /devices/pcx86/machine/5160/ega/640kb/debugger/machine.xml autoMount: A: name: PC DOS 2.00 (Disk 1) B: name: MS Word 3.0 (Disk 2) autoType: $date\r$time\rB:\rSPELL\r
This establishes a breakpoint on the “CALL 5” entry point inside the application’s Program Segment Prefix (PSP);
the breakpoint dumps the previous 10 instructions and then halts. The “uncompiled” version of PCx86 is being used
so that BackTrack™ information is available to the PCjs Debugger.
You can use the command
BC * to clear all predefined breakpoints and allow the program to run normally.
To start the machine, click “Run” or use the Debugger’s
And if all this seems like much ado about nothing, well, then put your feet up for a few and enjoy Nina Simone’s “I Put a Spell on You”, which hit #23 on the U.S. Billboard R&B chart in 1965. That way, your time here won’t be completely wasted.
May 27, 2018